This program is designed to study neuroanatomical correlates of control of onset of puberty in the rat. Various approaches to the problem will center around examination of the suprachiasmatic and preoptic nuclei, because these nuclei have been demonstrated to be of primary importance in control of the ovulatory surge of gonadotropins which is an essential element of puberty. From birth to first ovulation, changes in the length of dendrites, their branching and spines will be studied as represented in Golgi preparations. The maturation of neurons may also be shown by qualitative chances in the nucleolus. These will be examined in sections stained with thionin. Another correlary to maturation of the suprachiasmatic and preoptic nuclei will make use of H3-thymidine to study cell numbers in relation to hormonal events of maturation. In some experiments, onset of puberty will be altered by administering estrogens, and then the neuroanatomical changes will be followed to substantiate the importance of each anatomical correlate to the time of first ovulation. Tritiated amino acids will be used to study axonal efferents of the preoptic area and suprachiasmatic nuclei before and after puberty, in order to examine the brain for differences in axon terminal distribution. This program of studies seeks to provide an understanding of mechanisms of maturation in the brain, specifically those related to control of onset of puberty.